The disclosed methods and systems relate generally to a method of assembly of large battery packs from battery cells, and more specifically to assembly of high-energy battery packs from series-parallel arrays of small battery cells.
Applications of rechargeable batteries often require large battery packs having high voltages and high capacities. For example, battery packs used for power backup in telecommunications outdoor installations typically require 48 volts (V) and capacities of 50-1000 ampere-hours (Ah) to yield 0.25-5 kilo-Watt (kW) power outputs. Similarly, a battery pack capable of powering an electric automobile is typically required to have a voltage of 150-300 V and a capacity of 100-200 Ah to yield power outputs of 50-100 kW. Batteries capable of storing comparable amounts of energy are referred to herein as “large batteries”. Lead-acid and nickel-cadmium batteries are commonly used for these applications, and the battery packs are constructed by connecting a requisite number of large battery cells in series to satisfy the voltage requirements of the applications. Because these conventional batteries are based on an aqueous (i.e., water-based) electrolyte, they are considered safe systems, and large battery cells with tens to hundreds of ampere-hours capacity per cell can be built and used safely to assemble high energy battery packs of the type mentioned above. It is also known that because of their aqueous electrochemistry involving electrochemical reactions capable of equalizing the capacities of individual cells in a series string, the construction of large, high-voltage battery packs from aqueous battery cells does not require sophisticated battery management electronics (BME) to balance the capacities of individual cells in the string. Thus, construction of high-energy battery packs from large aqueous battery cells is a relatively simple process.
A practical disadvantage of aqueous batteries is their low energy densities, amounting to 70-200 Wh/liter, and low specific energies of the order of 30-70 Wh/kg, at the cell level. Many modern applications that call for very high power outputs and long operational times require batteries with significantly higher energy densities and specific energies. In this respect, Lithium-ion (“Li-ion”) batteries are an excellent choice by virtue of their very high specific energy of 180-200 Wh/kg and energy density of 450-500 Wh/liter at the cell level; however, large Li-ion battery packs have not yet been fully commercialized because of safety concerns and the cost of large battery cells.
The Lithium-ion battery is presently the power source of choice for portable consumer products the power requirements of which can be met by small battery packs, such as cellular telephones, personal digital assistants, video cameras, and notebook computers. These sample applications are filled by small commercial Li-ion cells, such as the industry standard “18650” cell type. In typical portable applications, small battery packs are constructed from several of these 18650 cells by connecting them in series and parallel as needed to meet the voltage and capacity requirements.
There are significant differences in the electrochemistries of conventional aqueous batteries and of Li-ion batteries that impact the construction of large battery packs from Li-ion cells. The electrochemistry of the non-aqueous electrolyte used in the Li-ion battery involves reactions that do not permit series/parallel stacking of cells to form batteries without electrolyte decomposition during charge and discharge cycling of such batteries, that is, unlike the aqueous battery systems, which are much more tolerant of over- and under-charging. Consequently, battery management electronics (BME) are needed to prevent overcharge and over-discharge of the Li-ion cells, in order to avoid electrolyte decomposition and failure of battery cells under these conditions. BME are also needed to balance the capacities of the individual cells in the battery pack during cycling. Accordingly, the assembly of battery packs from Li-ion battery cells require the use of sophisticated BME.
In the case of small battery packs used in cellular telephones and computers, the BME are relatively simple because the battery pack requires only one or two small cells stacked in parallel, or a few cells stacked in series and parallel. For example, a typical battery pack for a notebook computer has a voltage of 14.4 V and a capacity of 4 Ah, and is constructed from two parallel-connected strings, each of four 18650 size Li-ion cells stacked in series. Similarly, Li-ion battery packs used for powering cellular phones commonly have a nominal capacity of about one Ah and a nominal voltage of 3.6 V and can be constructed from a single Li-ion cell. In general, the BME required for Li-ion battery packs constructed from a few cells are of low complexity, irrespective of the size of the battery cells, and the complexity of the BME increases as the number of cells in the series/parallel assembly in the battery pack increases.
Thus, while the advantages of Li-ion batteries in terms of high power density are well-known, and while Li-ion batteries are well-accepted for small battery pack use, the art has not provided a suitable Li-ion battery construction that is suitable for fabrication of large batteries.